It is the purpose of this study to define the effects of retinoids on carcinogenesis in the tracheal epithelium. To achieve this goal, several areas are being simultaneously investigated. 1) Different retinoids are being screened in long term in vivo studies to determine the maximum dose and effects of the retinoids. 2) A reproducible model for the study of lung cancer using N-methyl-N-nitrosourea (NMU) in the hamster trachea is being developed. 3) Methods for collecting exfoliated cells from the lung are being investigated.